The invention relates to an ionization chamber with two electrodes and a radioactive source for ionizing the electrode gap, particularly for use in an ionization smoke detector.
Known ionization smoke detectors usually use two series-connected ionization chambers with different smoke sensitivities. For example, one of the chambers, normally called the measuring ionization chamber, can be constructed so as to be substantially accessible to air, while the other chamber, normally called the reference ionization chamber, is substantially shielded against the access of air or is sealed off from the atmosphere. Such ionization smoke detectors utilize the principle that, due to the attachment processes of the atmospheric ions formed by the radioactive source, when heavier particles such as smoke or other airborne combustion products penetrate the chamber, the ionic current flowing between the electrodes is reduced. As a result, the chamber resistance increases. As the reference ionization chamber is not affected, or is scarcely affected by smoke, its ionic current remains virtually constant, particularly if operated in the saturation range. Thus, the voltage drop across the measuring ionization chamber rises when smoke penetrates the chamber, and an evaluation circuit connected to the chamber emits an alarm signal if the voltage drop exceeds a predetermined threshold.
In practice, it is often necessary to change the threshold value and the sensitivity of such an ionization smoke detector in order to adapt them to ambient conditions. On the one hand, this can take place electrically by changing the evaluation circuit, and on the other hand it can be carried out by varying the ionic current or the resistance of one of the two ionization chambers. Various ionization smoke detectors are known in which the ionic current or the resistance either of the measuring ionization chamber or of the reference ionization chamber is changed by modifying the spacing between the two electrodes. Preferably, the reference ionization chamber is used, because in this case the geometrical conditions, and consequently the smoke sensitivity of the measuring ionization chamber, are only slightly affected.
However, in such known ionization chambers the change of spacing is preferably brought about by a screw to which the usually disc-shaped adjustable electrode is fitted. However, such constructions have proved to be mechanically unstable, particularly under the action of vibrations and impacts. In addition, the spacing and sensitivity cannot be adjusted with sufficient accuracy and precision. Furthermore, the ionic current change obtainable by such a modification of the spacing is smaller than that which would be theoretically possible, i.e. it does not have an optimum action. A further disadvantage is that such known adjusting mechanism take up a large amount of space outside the ionization chamber, and therefore, for example when used in an ionization smoke detector, can undesirably increase its total height.
The problem of the present invention is to eliminate the above disadvantages and provide an ionization chamber in which the ionic current or resistance can be adjusted in a reliable and optimum manner without the risk of any self-adjustment over a period of time under the influence of vibrations and impacts, whereby the space requirements are reduced and the stability and operating reliability increased.